The invention relates to a method for controlling the firing angle of a single-phase electric motor supplied with alternating current.
Electric motors nowadays may be activated largely independently of the mains frequency by way of electronic controls. However, even with the application of highly integrated electronic circuits, converter electronics create costs which often lie in the magnitude of that of the motor. For this reason, and in particular for motors of a small and medium power of below 200 Watts for example, one strives to provide controls which may be manufactured in a more economical manner and which operate the motor in a highly effective manner, i.e. at a high efficiency. In particular, permanent magnet motors which are provided with activation electronics constructed from triacs as power switches are suitable for this on account of their high efficiency. Such triacs which are allocated as switches to the individual motor windings have the advantage that they may be manufactured in an inexpensive manner, but have the disadvantage that with the usual activation it is the case of locking switches, i.e. which after switching on only block again when the current flowing through the switch becomes zero or changes its direction.
A method for the activation of a brushless electric motor by way of triacs is known from DE 35 07 883 A1 with which the mains voltage is connected to the part windings of the stator winding by way of phase-angle control such that one may also activate rotational speeds below the synchronous speed, but only certain rotational speeds which may not be freely selected.
A method for controlling a brushless electric motor is known from DE 35 17 570 A1, which likewise by way of triacs temporarily applies the mains voltage to the oppositely directed part windings of the motor winding, so that the motor may be activated at rotational speeds which lie above the synchronous speed by way of the production of opposing magnet poles. By way of this, the rotational speed may assume a multiple value of the synchronous speed, but only integer multiples of the synchronous speed.
It is thus known to activate an AC-supplied electric motor by way of triacs such that this may not only be operated synchronously, but supersynchronously and subsynchronously. As such, a further field of application opens up also to single-phase electric motors supplied with alternating current, which until now were only able to be controlled in rotational speed by way of extensive converter electronics.
However, phase-angle control as such is not sufficient in order to achieve a reliable start, a smooth running and a high efficiency on operation, and finally in order to avoid the magnets of the rotor becoming damaged due to an unallowably high magnetic field of the stator. This problem is described in DE 195 34 423 A1. For solving this problem, it is suggested there to provide a sensor for measuring the magnetic field of the rotor and with the help of phase-angle control to switch the alternating voltage to the stator winding of the motor depending on the magnetic field sensor signal, such that a moment is produced in the rotational direction. At the same time the current is limited in order not to produce any undesired high magnetic fields in the stator.
The disadvantage with this method is the fact that at least one magnetic field sensor is required for detecting the rotor position. Furthermore, the current limitation is achieved by way of measuring and integrating the currents and voltages and adapting them according to the rotor or the positions of the magnets.